Second generation biofuels: Thermochemistry of glucose and fructose

Combustion and Flame 157 (2010) 1230–1234

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Second generation biofuels: Thermochemistry of glucose and fructose A. Osmont a, L. Catoire b,c,*, P. Escot Bocanegra c, I. Gökalp c, B. Thollas d, J.A. Kozinski e a

DGA/Centre d’Etudes de Gramat (CEG), 46500 Gramat, France Department of Chemistry, Faculty of Sciences, University of Orléans, 1, Rue de Chartres, B.P. 6759, 45067 Orléans Cedex 2, France c C.N.R.S.—I.N.S.I.S., I.C.A.R.E., 1C, Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France d Polymaris Biotechnology, CCI de Morlaix, Aéroport, 29600 Morlaix, France e College of Engineering, University of Saskatchewan, 3B48 Engineering Building, 57 Campus Drive, Saskatoon, SK, Canada S7N 5A9 b

a r t i c l e

i n f o

Article history: Received 27 July 2009 Received in revised form 20 October 2009 Accepted 2 December 2009 Available online 25 February 2010

a b s t r a c t The energetic conversion of biomass into syngas or biogas is a more and more important topic. In the framework of these studies, improved understanding of glucose and fructose thermal decomposition and oxidation appears crucial. For this task, thermodynamic data are needed to make possible, for instance, the building of a detailed chemical kinetic model of glucose and fructose reactivity at high temperature. A semitheoretical protocol, presented elsewhere, is used for the estimation of the thermodynamic data of glucose and fructose in the gas phase. Five isomers of glucose and five isomers of fructose are considered and the lowest-energy conformers are found to be b-D-glucopyranose for glucose and bD-fructopyranose for fructose. The data for all 10 isomers are provided in the CHEMKIN-NASA format. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction First generation biofuels are bioethanol and biodiesel. They may not be totally ‘‘green,” but they address some of the drawbacks of current fossil fuels and their production is rapidly increasing. We have published studies dealing with the thermochemistry of biodiesel components, obtained from vegetable oils [1–3]. However, it is highly probable that first generation biofuels are not able to replace fossil fuels entirely. Furthermore, their production is expected to cause damage to the environment such as deforestation and intensive use of pesticides and is therefore questionable. Consequently, other fuel resources are needed. Second generation biofuels are obtained from biomass other than sugary or oleaginous plants, i.e., from plants that are not cultivated (wood), from agricultural residues (sugar cane and sugar beet residues, for instance), or from nonalimentary crops. Major constituents are cellulose, hemicellulose, and lignin; their amounts depend on the type of biomass. Biomass can be converted to syngas or biogas using various thermal processes. One of them is supercritical water gasification (SCWG) [4]. The chemistry of biomass in supercritical water might be complex and numerous molecules might form [4–6]. This is strongly dependent on the feedstock [7]. It is known that cellulose is converted into glucose and therefore the conversion of glucose to syngas has been studied thoroughly. Chemical processes in supercritical water are often interpreted in terms of * Corresponding author. Address: Department of Chemistry, Faculty of Sciences, University of Orléans, 1, Rue de Chartres, B.P. 6759, 45067 Orléans Cedex 2, France. Fax: +33 238696004. E-mail address: [email protected] (L. Catoire).

detailed chemical kinetic models as encountered in the high-temperature chemistry field (combustion, chemical vapor deposition, waste incineration, etc.), i.e., gas-phase chemistry [8]. The creation of these models requires knowledge of the thermochemistry of reactants, intermediates (radicals and molecules), and products [9,10]. For glucose conversion in supercritical water, products are numerous. CO2, CO, CH4, and H2 are identified in the gas phase and 23 compounds (acetic acid, propanoic acid, etc.) are identified in the liquid phase [6]. Some reaction pathways have been proposed to explain the conversion of glucose. Most of these reactions are global ones, whereas elementary reactions are required to explain fundamentally the course of the conversion of glucose. It is, however, premature to propose a detailed chemical kinetic model constituted of elementary reactions, and the proposal of global reactions is therefore highly justified. The first step in the proposal of a detailed chemical kinetic model of the conversion of glucose or fructose in a thermal process, whatever it is, is the establishment of the thermochemistries of glucose and fructose in the gas phase. The existence of glucose in the gas phase has not been assessed. The same holds for fructose. There is a vapor pressure above solid glucose, but the chemical nature of this vapor has not been reported [11]. For modeling purposes the following sequence can be assumed:

liquid glucose ! gaseous glucose ! intermediates ! products: Once the thermochemistries of glucose and fructose in the gas phase are known, it is then possible to discuss bond breaking in the molecules and propose elementary reactions able to explain the formation of products.

0010-2180/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2009.12.002

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A. Osmont et al. / Combustion and Flame 157 (2010) 1230–1234 Table 1 Atomic corrections used in the present model. Atom

ci (hartree atom1)

H C O

0.581896 38.115345 75.150410

2. Computational methods Theoretical methods have been shown to be reliable tools for estimating the thermochemistry of many compounds [12]. In previous studies [1–3,9,13], we presented a B3LYP/6-31G(d,p) method using an atomization approach to allow the estimation of thermodynamic data for large molecules and large radicals for which highly accurate G2 or G3 methods, and their sequels, are inappropriate. The protocol used here is useful for molecules for which experimental data or previous higher level calculations do not exist. The method employed in the present study is based on compounds having well-calibrated enthalpies of formation, with uncertainty less than 1 kcal/mol for most of them. The compounds considered are general organic compounds from all the chemical families, including polyfunctional compounds. This was found to be a good compromise between computation time and accuracy. It is beyond the scope of this paper to give the details of this pro-

Table 2 Chemical structures of glucose isomers (C6H12O6) considered in this study. Compound

OH

The method developed and implemented in the present study is validated for a number of compounds, including almost all the chemical families and polyfunctional compounds. For the present study, compounds of interest are reported in Table 2 (isomers of glucose) and Table 3 (isomers of fructose).

Compound

O HO

3. Model validation

Table 3 Chemical structures of fructose isomers (C6H12O6) considered in this study.

Structure

b-D-Glucopyranose

tocol, which are still available in the literature [1–3,9,13]. Three fundamental thermochemical properties are needed for combustion, oxidation, or thermal decomposition modeling: standard gas-phase enthalpy of formation at 298.15 K, DfH°298.15 K, standard  heat capacities at constant pressure against temperature, CpT , and  standard absolute entropies against temperature, ST , and generally these data are needed in a wide temperature range (typically from ambient up to 5000 K for biomass energetic conversion using thermal plasmas [13–15]). Enthalpies of formation of species, in particular molecules, can be estimated quite conveniently using group additivity methods [16–18]. However, these estimates are not always feasible due to the lack of some group values and of some ring strain constraints. This hampers the estimation of the thermodynamic data for radicals formed from parent molecules and can hamper the development of detailed chemical kinetic models. Calculations were carried out using the Gaussian 98 [19] and Gaussian 03 [20] computational chemistry programs. Table 1 gives the atomic corrections used in this study for C, H, and O atoms. In this paper, we report data for the lowest-energy conformers.

OH

HO OH

a-D-Glucopyranose

Structure

b-D-Fructopyranose

HO O HO

OH

OH

O HO

OH OH b-D-Glucofuranose

OH

OH

HO

a-D-Fructopyranose

HO

OH HO

O

OH O OH

OH OH

OH

OH

b-D-Fructofuranose

HO

OH

O OH

HO a-D-Glucofuranose

OH

OH HO

OH a-D-Fructofuranose

O

HO

OH HO Open-chain D-form

O H HO

OH OH OH

Open-chain D-form

OH H

O HO

H

H

OH

H

OH

H

OH

H

OH

OH

OH

O HO

OH

OH

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A. Osmont et al. / Combustion and Flame 157 (2010) 1230–1234

Table 4 Comparison between experimental and calculated standard enthalpy of formation at 298.15 K for heterocyclic oxygen compounds. Compound

O

Experimental value

Calculated value

Deviation exp-calc

12.58 [23]

13.6

1.02

22.63 ± 0.15 [23]

24

1.37

72.10 ± 0.53 [23]

72.10

0

126.2 ± 0.41 [21]

127.2

1

44.03 ± 0.17 [23]

44

75.36 ± 0.19 (23) 76.0 ± 0.4 [23]

76.9

35.43 ± 0.40 [23]

34.5

0.93

53.39 ± 0.37 [23]

53.3

0.09

19.25 ± 0.15 [23]

20.86

1.61

30.62 ± 0.23 [24]

29.77

0.85

Ethylene oxide. C2H4O O H3 C

Propylene oxide. C3H6O O

O

1,3-dioxolane. C3H6O2 O

O

O

Dihydro 2,5-furandione. C4H4O3 O

0.03

Tetrahydrofuran (THF). C4H8O

O

0.9 up to 1.54

O

1,4-dioxane. C4H8O2 CH 3 O CH 3

3,3-dimethyl oxetane. C5H10O

O

Tetrahydropyran. C 5H10O O

Oxetane. C3H6O H3 C

O

CH 3

2,5-dimethylfuran (DMF). C6H8O

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A. Osmont et al. / Combustion and Flame 157 (2010) 1230–1234 Table 4 (continued) Compound

O

Experimental value

Calculated value

Deviation exp-calc

8.29 [23]

8.27

0.02

87.38 ± 0.11 [23]

89.4

2.02

Furan. C4H4O O O

γ-butyrolactone. C4H6O2 Note: Calculations have been performed using the theoretical protocol described in the paper.

Table 5 Calculated standard enthalpy of formation at 298.15 K of gas-phase glucose isomers. Compound

Gas-phase standard enthalpy of formation at 298.15 K (kcal/mol)

b-D-Glucopyranose

249.7 247.5 246.6 243.4 242.9

a-D-Glucopyranose a-D-Glucofuranose b-D-Glucofuranose Open-chain D-form

Note: Calculations have been performed with the protocol described in this paper.

Table 6 Calculated standard enthalpy of formation at 298.15 K of gas-phase fructose isomers. Compound

Gas-phase standard enthalpy of formation at 298.15 K (kcal/mol)

b-D-Fructopyranose

248.8 247.9 238.6 244.9 241.2

a-D-Fructopyranose a-D-Fructofuranose b-D-Fructofuranose Open-chain D-form

lowest-energy conformer is b-D-glucopyranose and then a-D-glucopyranose. a-D-Glucofuranose, b-D-glucofuranose, and the openchain form are higher in energy. For fructose in aqueous solution, major forms are b-D-fructopyranose and b-D-fructofuranose [22]. As shown in Table 6, in the gas phase, the lowest-energy conformer is b-D-fructopyranose and then a-D-fructopyranose. a-D-Fructofuranose, b-D-fructofuranose, and the open-chain form are higher in energy. Vibration frequencies (scaled by 0.9613) and moments of inertia have been calculated for all these species. These data are reported as Supporting informaion. From these data, Cp° and S° have been calculated up to 5000 K. This temperature range is certainly too wide for SCWG, but it can be useful for other thermal processes, such as thermal plasma processes. The thermochemical data computed here are provided in the CHEMKIN-NASA format as Supporting informaion. This will help modelers’ work and will allow thermodynamic calculations at equilibrium.

5. Conclusions

Note: Calculations have been performed with the protocol described in this paper.

In solution in water, the glucose molecule exists in a linear form in equilibrium with five-membered and six-membered heterocyclic forms. It seems necessary therefore to validate the computational protocol with oxygen heterocyclic compounds such as oxetane and tetrahydropyran. The corresponding validation is given in Table 4. The accuracy provided by the protocol presented in this paper is estimated to be about 3 kcal/mol, depending on the chemical family. This is based on results obtained in the framework of the calibration and validation procedures of the protocol. The occurrence of systematic biases for specific classes of compounds (nitrates, highly strained polycyclic alkanes) is possible and has been examined elsewhere. For general C/H/O compounds, the model does not seem consistently biased for a particular group. This is also the case for heterocyclic oxygen compounds presented in Table 4, for which the average absolute deviation is better than 3 kcal/mol.

4. Results Data obtained using the protocol presented in this paper are reported in Table 5 for glucose and Table 6 for fructose. In solution in water, as is observed during the mutarotation of glucose, the two major forms of glucose are b-D-glucopyranose and a-D-glucopyranose [21]. The open-chain form and the furanose forms also exist, but in minor quantities. As shown in Table 5, in the gas phase, the

A theoretical protocol is used for the estimation of the thermodynamic data of gas-phase heterocyclic oxygenated compounds including glucopyranose, glucofuranose, fructopyranose, and fructofuranose. These data are needed for thermochemical calculations performed to assess the feasibility of a process and for the writing of detailed chemical kinetic models dealing with the gasification of biomass. Generating these enthalpies of formation is a first step. It is then possible from these data to determine the thermochemistry of radicals formed from glucose and to determine the preferred reaction pathways of glucose thermal decomposition in various environments (temperature, pressure, composition). Numerous species need to be considered in particular biradicals. The ability of the protocol used here to provide data for biradicals remains to be demonstrated. This work is in progress.

Acknowledgment The authors thank the French National Research Agency (ANR), Pôle National de Recherche sur les Biocarburants (PNRB) for its financial support in the frame of the SUPERBIO project.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2009.12.002.

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